Quorum Race: New-Then-Old, Then Write-Back Diagram

Partial is not acknowledged

The write targets w=3 but only n1 stores v2: partial, never acknowledged
Reader A sees {n1:v2, n2:v1} and returns new; reader B starts later, sees {n2:v1, n3:v1}, returns old
w + r > n still holds as arithmetic; the freshness premise (a completed write) never existed

Repair after return is too late

Background read-repair runs after the read returns: too late for linearizability
The fix writes the max version back to a quorum BEFORE the read returns
After write-back, every later quorum meets v2: new-then-old becomes impossible

Versions, writers, membership

Ordered versions (v1 < v2) let readers pick the newest without a leader
With concurrent writers, add a query phase: read the highest timestamp before writing
Membership is fixed here; changing members need overlapping old/new quorums